UV-hardenable dispersions according to the prior art very often contain polyester acrylate components as essential structural units. These polyester structures are potential weak points, since ester bonds are inherently susceptible to hydrolysis. Other dispersions according to the prior art contain larger amounts of polyether constituents, either incorporated in the polymer or as external substances acting as emulsifier or stabiliser. Such polyether structures, especially if they are contained in relatively large amounts, often lead in the case of coatings to increased sensitivities, for example to colouring liquids, and also have disadvantages as regards their weather resistance, including chalking (whitening) of the coating.
In EP 1 118 627 A1 weather-resistant, water-dispersible and radiation-hardenable polyurethanes are described, synthesised substantially from aliphatic or alicyclic polyisocyanates, cycloaliphatic diols and/or cycloaliphatic diamines, compounds containing unsaturated groups, and compounds with a dispersion-active group. The polyurethanes contain short-chain, cycloaliphatic diols or diamines as essential synthesis component. Methylene- or isopropylidene-bis(cyclo-hexylamine), piperazine, 1,4-diaminocyclohexane or 1,4-bisaminomethyl-cyclohexane are named as possible cycloaliphatic diamines. As hydrophilic component these comprise exclusively ionic-hydrophilising structural units. The disadvantage of the polyurethanes disclosed there is their high proportion of cycloaliphatic diols. This leads to coatings which have only a very limited elasticity. Such coatings are as a rule not suitable for use on wood or plastics. Also, the reactivity of these dispersions is not always sufficient for many applications, especially in the industrial wood and furniture coating sector. The pendulum hardnesses disclosed in the Examples (Example 1: 114 pendulum seconds) are not particularly high and are not always adequate for wood coatings.
WO 01/23453 A1 describes aqueous polyurethane dispersions that can be hardened by UV radiation and heat, which apart from double bonds polymerisable by UV radiation also contain blocked polyisocyanate groups and in addition free hydroxyl groups. Such dual cure systems require elevated temperatures, as a rule above 100° C., for the hardening by reaction of the blocked polyisocyanate groups, as a result of which their use for temperature-sensitive substrates such as wood or plastics is greatly restricted or is impossible. The document discloses the coating of thermostable substrates such as metallic substrates for automobile painting. A chain extension reaction with diamines or polyamines for synthesising high molecular weight polyurethanes is not disclosed.
EP 0 704 469 A2 discloses radiation-hardenable, aqueous polyurethane dispersions based on polyester polyols with a specifically generated, broad molecular weight distribution. Structural units for the chain extension are disclosed only as an optional component.